1. Field of the Invention
The present invention relates to an ink jet printing method and printing system that prints on a print medium using colorant-containing ink. More specifically, the present invention relates to a printing method that uses an ink jet print head having a plurality of nozzles arrayed at high density.
2. Description of the Related Art
As technologies associated with copying machines, information processing devices such as word processors and computers, and communication devices advance, ink jet printing apparatus have come into widespread use as a means to record digital images from these devices. The ink jet printing apparatus use a print head that has a plurality of printing elements (nozzles) highly densely arrayed, each made up of an ink ejection opening and a liquid path for supplying ink to the opening. Printing is done by the print head ejecting ink, a printing liquid, onto a print medium such as paper. The ink jet system has an advantage of low noise because of its non-contact operation. Another advantage of the ink jet system is that it can realize a high-resolution printing relatively easily by increasing the density of nozzles and, even with low-cost print media, such as plain paper, can form high quality images without requiring special processing, such as development and fixing. Especially an on-demand type ink jet printing apparatus can easily be made to perform color printing and the apparatus itself can be made small in size and simplified, offering a bright prospect in meeting future demands.
The ink jet printing apparatus can be grouped largely into two types: serial type and line type. In the serial type printing apparatus, an image is progressively formed by repetitively alternating a main scan operation, in which a print head having a plurality of nozzles arrayed in a print medium feeding direction is moved in a direction crossing the print medium feeding direction as it prints, and a sub-scan operation, in which the print medium is fed a predetermined distance in relation to a width of a strip of area printed by the main scan. The serial type ink jet printing apparatus is characterized by its relatively small size and low cost.
In the line type printing apparatus on the other hand, an elongate print head (line type elongate print head) having nozzles arrayed in a line longer than a width of an image to be formed is used and the print medium is moved relative to the print head in a direction crossing the nozzle array direction to form an image. Therefore, compared with the serial type printing apparatus that performs the printing scan operation many times, the line type printing apparatus can form an image much faster. There are increasing demands on the ink jet printing apparatus for higher image quality and faster printing speed and, to meet these requirements, efforts are being made to develop a technology for integrally fabricating nozzles in the print head at high density. Under these circumstances expectations are growing for a printing apparatus equipped with such a line type elongate print head.
However, new problems have surfaced with the ink jet printing apparatus capable of printing high-resolution images at high speed.
In a printing system, whether of a line type or a serial type, that forms an image on a print medium with one or a small number of printing scans, as when performing printing at high speed, it is desired that the print medium absorb and fix a predetermined volume of ink ejected from multiple print heads in a relatively short period of time. However, some kinds of print media cannot completely absorb the predetermined volume of ink in a predetermined length of time. Should ink that failed to be absorbed instantly remain on the print medium, ink droplets that have landed at adjoining positions will come into contact and merge together, resulting in the adjoining ink droplets pulling one another or causing a color mixing on the surface of the print medium, which in turn degrades an image quality.
A possible countermeasure to get around this problem may include the use of a fixing device with heating and drying functions. This, however, makes the apparatus bigger and costly. For the ink jet printing apparatus featuring a low cost, this is not a practical solution. Another method may involve reducing the amount of ink to be used for printing in order to quicken the fixing of ink. This, however, gives rise to another problem that images obtained have lower densities and printing resolutions than required.
We will explain in the following various image problems that appear when an ink absorption speed or rate is slow and some methods for solving these problems.
Japanese Patent Application Laid-open No. 6-40046(1994), for example, discloses a printing method which prevents the formation of unintended lines that appear at boundaries between adjoining printing scans in a serial type ink jet printing apparatus. Ink droplets ejected from one end of the print head in each printing scan are recorded at positions adjoining those ink dots formed during the previous scan. If the ink absorption rate is slow, ink droplets ejected from one end of the print head come into contact with the ink droplets that were recorded on the print medium in the previous scan, thus affecting their positions. As a result, a white or dark line called an interface or boundary line may show up at a boundary between printed image strips of successive printing scans. Japanese Patent Application Laid-open No. 6-40046(1994) discloses a method of solving the boundary line problem by differentiating a print timing at the boundary from those of other areas.
Japanese Patent Application Laid-open No. 2000-118007 discloses a technique to produce solid printed images forming characters and graphs at high contrast without spreading ink. If an ink absorbing rate is slow, particularly when an image being formed has ink locally concentrated, as in characters and graphs, the ink may flow from edges of printed areas into unprinted areas, blurring a boundary of the image. To deal with this problem, Japanese Patent Application Laid-open No. 2000-118007 describes a technique that extracts solid image areas that are printed at 100% duty, classifies them into boundary pixels and skeleton pixels and ejects a larger volume of ink onto the skeleton pixels. With this technique, the overall density of the solid image area can be raised by the skeleton pixels while at the same time the ink overflowing from the skeleton pixels can be absorbed by the surrounding boundary pixels, thus preventing excess ink from flowing out into the outer non-printed areas and producing a high-contrast image with clear edges.
In printing a highly detailed image at a high grayscale level, image processing up to the transforming of an original image signal into a binary signal that the printing apparatus can use for printing plays an important, complicated role. The image processing performs an image data conversion to produce a desired density, considering a state of ink that has landed on a print medium. Thus, a behavior of ink on the print medium is preferably stable so that it is predictable to some extent. The more stable the behavior of ink on the print medium, the more easily a desired level of grayscale can be produced and the higher the reliability of the image processing will become.
However, when a printing is performed on a print medium whose ink absorption is slow, the shape of large dots that are formed by the adjoining dots merging together is strongly influenced by the presence or absence of adjoining dots, a time difference between adjoining dot landings, and subtle variations of dot landing positions. Hence, even if printing is done according to the same image signal, the dots on the print medium are not stable in their shape, with the resultant grayscale having low reproducibility. Further, the merging of dots may produce artifacts having density discontinuity. That is, even when attempting to form an image with smooth tonal gradation, sudden local density variations appear at certain grayscale levels, forming streaks or line artifacts which look like white or dark lines, degrading the image quality.
What is required of the current ink jet printing apparatus is to realize a further image quality improvement, a higher speed and a reduced cost. To this end one of the most important tasks is to solve the problems associated with the image processing described above and thereby achieve a reliable, stable grayscale and color reproduction capability. However, with conventional technologies including the above-cited patent references, although problems, such as boundary line and character print quality problems that arise when the ink absorption is slow and problems associated with objects formed in solid image areas, can be dealt with individually, no methods have been available that can solve other problems positively and comprehensively. That is, a printing method capable of realizing a reliable, stable grayscale and color reproduction has yet to be proposed.